1. Field of the Invention
The present invention relates to a injection control system for a turbocharged diesel engine and more specifically to such an injection system which improves injection control during acceleration type modes of operation.
2. Description of Prior Art
SAE Paper 860145 and JP-U-63-177638 disclose electronic control systems for varying the injection timing and amount by distribution types of fuel injection pumps.
FIG. 10 shows such a distribution fuel injection pump 1 wherein a drive shaft 4, which is adapted for connection with an output shaft of diesel type internal combustion, is arranged to drive a vane type feed pump 2. Fuel is inducted from a non-illustrated source into a pump chamber 5 and transferred via passage 6 into a plunger chamber 12 of a plunger pump 3.
One end of the drive shaft 4 is connected to a cam/roller arrangement (elements 9, 9a, 9b, 10, 11) which is located at one end of a plunger 7 and which translates the rotary movement of the drive shaft 4 into both reciprocal and rotational movement of a plunger 7.
The plunger is formed with a plurality of grooves 8 which enable fuel to be forced via a non-illustrated delivery valve to the respective injection nozzles.
The arrangement further includes a rapid acting electromagnetic injection control valve 14. This rapid acting valve 14 is arranged with the plunger pump in a manner wherein, that during the compression phase of the pump, when the valve 14 is closed, injection is initiated and when the valve is opened injection is terminated. In other words the initiation and termination of the injection is controlled by this valve. By controlling the period between the initiation and the termination, the amount of fuel injected can be controlled.
The operation of this valve is controlled by a non-illustrated control unit which includes a microprocessor and which is supplied with plurality of data inputs. These inputs take the form of an engine speed, accelerator pedal depression, engine coolant temperature, fuel temperature and the like.
A ROM which forms part of the above mentioned control unit microprocessor includes a program for determining the initiation and termination timing (and hence the injection amount) in accordance with the operating conditions of the engine.
During actual operation of this arragnement two pulse trains are produced. As shown in FIG. 11 the first contains on reference pulse per rotation of the pump while the second contains thirty six scale pulses. In accordance with the engine speed, the accelerator pedal depression degree, the engine coolant and fuel temperatures, a basic fuel injection amount is read out of memory and used in combination with the above mentioned to produce a valve actuation pulse which is supplied to the valve 14.
However, in the vent the above type of injection control arranged is applied to a turbocharged type diesel engine, a problem has been encountered in that the turbocharger requires a finite time to respond to changes in engine operation/load (viz., exhibits so called turbo lag). That is to say, when the engine is subject to sudden acceleration such as vehicle movign away from from standstill or immediately following a gear change, the delay in the turbocharger response (turbo lag) induces a corresponding delay in amount of air which is supplied to the engine being increased to the desired level. At this time, despite the delay in boost from the turbocharger, the amount of fuel injected is controlled in accordance with the amount of accelerator pedal depression. This brings about the formation of an excessively rich air-fuel mixture and leads to the formation of smoke and a reduction in combustion efficiency.
In order to overcome this problem it has been proposed in JP-A-60-104743 to detect the presence of engine acceleration and to reduce the formation of smoke by reducing the maximum load injection amount. An alternative proposal disclosed in JP-A-62-223423 has included sensing the pressure prevailing downstream of the turbocharger compressor and the engine and determining a first injection amount which is selected to generate a limited amount of smoke, and a second amount which is determined using the accelerator pedal depression egree and selecting the optimum injection amount for non-transitory engine operation. The first and second amounts are compared and smaller is used for actual injection control.
Nevertheless, as the amount of air per se which is actually inducted is not directly sensed, it is very difficult to determine just how much air is actually charged into the engine cylinders and how much to reduce the injection volume. As a result an inevitably deterioration in the engine output characteristics has resulted.
Further, as different from non-transitory engine operation, during acceleration, the rise in exhaust pressure prevailing upstream of the turbocharger turbine (hereinafter referred to as exhaust pressure) occurs before the supercharge pressure increases, a pressure pressure differential develops between the exhaust and supercharge pressurse.
During sudden acceleration, the pressure differential tends to maximize and the charging efficiency tends to deteriorate with the degree of the acceleration. As a result, if the fuel amount is controlled in accordance with the supercharge pressure only, depending on the degree of acceleration, while the amount of smoke is reduce it is impossible to simultaneously prevent reductions in power output and fuel economy.
It should be noted that FIGS. 12 to 14 show respectively in terms of engine speed and pressure head, engine speed and supercharge pressure PB and engine speed and exhaust pressure P.sub.EX, the relationships which develop between above mentioned parameters with turbocharged type diesel engines.